The present invention relates generally to semiconductor device processing techniques, and, more particularly, to a structure and method of forming silicon-on-insulator (SOI) trench memory with a single-sided buried strap.
Dynamic random access memory (DRAM) cells include two main components, a storage capacitor that is used to store a binary data bit in the form of an electric charge and an access transistor that is used to transfer the electric charge to and from the storage capacitor. Typically, a first voltage is stored on the capacitor to represent a logic HIGH or binary “1” value (e.g., VDD), while a second voltage on the storage capacitor represents a logic LOW or binary “0” value (e.g., ground). The storage capacitor may be either planar on the surface of the semiconductor substrate or trench etched into the semiconductor substrate. In the semiconductor industry (where there is an increased demand for memory storage capacity accompanied with an ever decreasing chip size), the trench storage capacitor layout is favored over the planar storage capacitor design since this particular arrangement results in a dramatic reduction in the space required for the capacitor without sacrificing capacitance.
One of the more delicate aspects of DRAM cell fabrication is the electrical connection made between the trench storage capacitor and the access transistor. Such a contact is often referred to in the art as a “buried strap,” which is formed at the intersection of one electrode of the storage trench capacitor and one source/drain junction of the access transistor. In a “cross-point” DRAM array configuration, bit lines and word lines crisscross the array in a perpendicular fashion such that a storage capacitor is located at each word line/bit line intersection. However, each storage capacitor in a cross-point array typically must be accessible from only one side of the deep trench; otherwise, the “one word line, one bit line, one memory bit” rule would be violated. Thus, an interconnection is formed between the storage capacitor and a vertical sidewall portion of the deep trench, while the deep trench is isolated along all other sidewall portions.
Present approaches to forming a single-sided buried strap include (in part) performing angled ion implantation into a layer of deposited silicon within the deep trench, thereby leaving the silicon layer one side of the deep trench un-implanted. The un-implanted silicon is then etched using an etch selective material with respect to the doped (implanted) portion of the silicon, which thereby acts as a sacrificial mask that is subsequently removed.
One drawback associated with angled ion implantation is the complexity associated therewith, given the deposition and etching of multiple sacrificial materials. In addition, process control also becomes a factor in the angled ion implantation approach to single-sided strap formation. Accordingly, it is desirable to be able form trench memory devices on an SOI substrate (for example) with a single-sided strap in a cost effective and robust manner.